1. Field of the Invention
The present invention relates to a liquid crystal optical modulator, and more particularly to a liquid crystal optical modulator which allows an adaptive optics technology to be used for maintaining a reliable optical communication link in free space optical communication between satellites, between ground station and satellite, or between ground stations, which can operate as a concave lens or a convex lens, and which can vary the operation of a concave lens and a convex lens.
2. Description of the Prior Art
To increase the laser communication speed, it is necessary to keep a per-bit light volume constant and to increase the whole light volume. To do so, it is important to increase the antenna gain of a transmitter and a receiver. In a free space optical communication such as a communication between a ground station and a satellite, an atmospheric turbulence in an optical transmission line disturbs the wavefront of a laser beam that propagates through air. One solution to this problem is to increase the antenna diameter to increase the antenna gain. However, the problem is that, even if the antenna diameter is increased to some degree, a wavefront distortion due to an atmospheric turbulence prevents antenna directivity from being improved, and the antenna gain from exceeding a predetermined value.
Therefore, for a high-speed optical communication in a free space, it is necessary to compensate for an atmospheric turbulence. Recently, an optical communication technology for compensating for an atmospheric turbulence using the adaptive optics technology is under development.
In the optical communication technology based on the adaptive optics technology, a deformable mirror is controlled using wavefront distortion data, detected by a wavefront sensor, to reconstruct a wavefront free from an atmospheric turbulence.
As described above, detecting a wavefront distortion using the adaptive optics technology is an important factor. One example of wavefront detection sensors is a Shack Hartmann sensor. FIG. 32 shows the principle of the Shack Hertmann sensor. An incoming ray 2701, which has a wavefront distortion, is divided by a micro lens array 2703, and an image is formed at a focus 2707 on a CCD array 2705. The position of this focus 2707 can be calculated from the peak value of an intensity distribution curve 2709 obtained by calculating the light intensity detection value of the CCD array 2705. That is, the positional displacement of this focus 2707 is determined by detecting the light intensity.
This positional displacement corresponds to the wavefront tilt of each component of the incoming ray 2701 created by dividing by a small aperture of each micro lens. Because this tilt information is proportional to the linear differential of the wavefront, the calculation is performed based on the information for reconstructing the wavefront free from an atmospheric turbulence.
In this case, the diameter and the focal length of a micro lens affect the reconstruction precision of the wavefront. That is, the focal length determines the precision at which the wavefront tilt of the measuring system is detected. In addition, because the diameter of a micro lens affects the brightness at the focus and the spatial resolution of the wavefront tilt to be measured, a higher special resolution requires a smaller diameter.
The micro lens array 2703 described above is of fixed focus type. However, because the wavefront distortion depends largely on the measurement location and the season, multiple types of fixed-focus micro lens array must be prepared for exchange of the array according to the degree of a wavefront distortion to be measured. The problem with exchanging the lens array is that the device becomes large and the time is required for the exchange. It is therefore desirable that the micro lenses forming the micro lens array be of variable-focal length type.
As a method for implementing a micro lens array for use in the application described above, the inventor of the present invention has already proposed a technology for arranging liquid crystal optical modulators, each having the configuration described below for use as a lens, in an array form to allow the array to function as the micro lens array described above (for example, Japanese Patent Laid-Open Publication No. 2000-214429).
Unlike the liquid crystal optical modulator described above, a predetermined voltage is applied across a plurality of stripe electrodes in this configuration using a quadratic curve approximation area of a liquid crystal phase modulation area. This configuration allows the liquid crystal optical polarimeter to function as a convex lens. The configuration and the operation will be described below.
First, the following describes the configuration of a liquid crystal lens of the liquid crystal optical modulator in this system. FIG. 1 is a cross section diagram showing the configuration of a liquid crystal optical modulator used in a micro lens array of variable-focal length type.
As shown in FIG. 1, the liquid crystal optical modulator comprises a first substrate 103 on which a composite electrode 111 is formed; a second substrate 105 on which a full-area opposed electrode 113 is formed; and a nematic liquid crystal layer 101 held between the two substrates. The nematic liquid crystal layer 101 forms an alignment layer 117 on the composite electrode 111 of the first substrate and on the opposed electrode 113 of the second substrate. This alignment layer 117 homogeneously aligns directors 107 of p-type liquid crystal molecules, each with a tilt angle 109 of 0.5 to 20 degrees when no electric field is applied. A non-reflecting coating 115 for preventing reflection is provided on the opposite side of the liquid crystal layer of the first substrate 103 and the second substrate 105, respectively.
Conventionally, a configuration is known in which the liquid crystal optical modulator described above is used to form a cylindrical lens. With reference to FIG. 33, the structure of the composite electrode 111 for forming a cylindrical lens using a liquid crystal optical modulator will be described in detail.
FIG. 33 is a top view of the composite electrode 111. The composite electrode 111 has two lens areas in an active area 2871: first cylindrical lens area 2851 and second cylindrical lens area 2861.
The first cylindrical lens area 2851 and the second cylindrical lens region 2861 form a stripe electrode bundle composed of a first stripe electrode 2820 to the Nth stripe electrode 2829 (N=10 in FIG. 33) and from the (N+1)th stripe electrode 2830, which is the first electrode of the second cylindrical lens, to the 2Nth stripe electrode 2839, all of which are formed by a low-resistance polycrystalline transparent conductive film such as an ITO (Indium Tin Oxide) film. This stripe electrode bundle is connected by a first gradient potential electrode 2801. Although the first gradient potential electrode 2801 may be formed at the same time the stripe electrodes are formed using the same material, it is desirable that the gradient potential electrode be formed by an amorphous conductive material, which is transparent and has a resistance higher than the ITO film of the stripe electrodes, such as a material created by adding a predetermined amount of impurities to In2O3.
A signal electrode a 2811, a signal electrode b 2813, and a signal electrode c 2815, each composed of low-resistance metal materials such as Mo and Ag alloy, are connected to the both ends and to the center of the first gradient potential electrode 2801, respectively. The opposed electrode (not shown) is a full-area electrode formed by an ITO film.
FIG. 34(a) is a perspective view showing an example of the configuration of a cylindrical lens. The composite electrode 111 provided on the first substrate 103 and the full-area opposed electrode 113 provided on the second substrate 105 hold the nematic liquid crystal layer 101 between them and refract the light passing through the electrodes and liquid crystal to form a lens.
In this configuration, AC pulse signals, with equal amplitude and frequency but 180 degrees out of phase to one another, are applied to the signal electrode a 2811 and c 2815 provided on the gradient potential electrode 2801 in the liquid crystal optical modulator described above, and 0[V] is applied to the signal electrode b 2813. This causes the gradient potential electrode 2801 to create a linear potential gradient. This linear potential gradient allows each two neighboring stripe electrodes to have incremental potential gradients. FIG. 35(a) shows a gradient potential formed among signal electrodes a, b, and c. In this way, the stripe electrode bundle generates a potential distribution with a linear gradient in the liquid crystal optical modulator.
Thus, for example, when two lens areas are used for one lens, a convex lens area can be formed by the two lens areas: first cylindrical lens 2851 and second cylindrical lens 2861. The numeral 2901 in FIG. 34(a) schematically illustrates this convex lens area.
Next, the characteristics of the conventional liquid crystal layer and the operation area of a liquid crystal optical modulator will be described. The wavefront of an incoming linearly polarized light received by a liquid crystal optical modulator, which employs homogeneous alignment, is modulated according to the characteristics of applied voltages versus effective birefringences such as the one shown in FIG. 5.
In FIG. 5, the horizontal axis indicates the voltage V applied to the liquid crystal layer, and the vertical axis indicates the effective birefringence Δn. The shape of the electro-optic response curve shown here depends on such factors as the elastic constant of the liquid crystal that is used, the dielectric anisotropy, and the pre-tilt angle determined by the alignment layer when no electric field is applied. In addition, because the refractive index and the birefringence of the liquid crystal layer depend on the wavelength, the electro-optic response curve also varies according to the wavelength of the light source.
With reference to the characteristic diagram shown in FIG. 5, the operation areas will be described wherein the area from the liquid crystal voltage 0 [Vrms] to the first inflection point is a first linear area 521, the curve area from the first inflection point to the second inflection point is a first quadratic curve approximation area 520, the curve area from the second inflection point to the third inflection point is a second quadratic curve approximation area 522, and the area from the third inflection point to the high-voltage side area is a second linear area. Note that the second linear area is not shown in FIG. 5.
The conventional liquid crystal optical modulator performs the convex lens operation with the vicinity of the first quadratic curve approximation area 520 as the convex lens curve area.
FIG. 5 shows the electro-optic response curves generated by setting the pre-tilt angle to 10°, 5.0°, 2.0°, and 0.5°, respectively. As shown in FIG. 5, when the pre-tilt angle is set to 0.5°, the area from the liquid crystal voltage from 0 to 1 [Vrms] is the first linear area 521. Therefore, in this voltage range, it is difficult to use the liquid crystal optical modulator as a spherical lens.
On the other hand, for the characteristic curves of other pre-tilt angles, the figure shows that the liquid crystal optical modulator can be best used as a spherical lens approximately in the liquid crystal voltage range 0–2[Vrms]. This characteristic curve varies according to the material of the liquid crystal or the liquid crystal film thickness. In any case, the liquid crystal advantageously operates as a phase modulation layer except for the pre-tilt angle of 0.5°, especially in the first quadratic curve approximation area indicated by the numeral 520 in FIG. 5. Preferably, the pre-tilt angle at this time should be 2 to 10 degrees.
The following describes an example of the operation of a convex lens made of a liquid crystal lens in the first quadratic curve approximation area described above. For example, assume that the opposed electrode 113 in FIG. 1, which is a full-area electrode, and the signal electrode b 2813 are set to 0[V], that +V[V] that is the voltage of the first quadratic curve approximation area 520 is applied to the signal electrode a 2811, and that −V[V] is applied to the signal electrode c 2815. Then, as shown in FIG. 35(a), the first gradient potential electrode 2801 provided on the composite electrode 111 has a potential gradient.
The stripe electrode bundle, to which a ramped potential gradient is applied, forms a linearly ramped potential distribution in the liquid crystal layer 101. Because of this potential distribution, the value of the effective refractive index of the liquid crystal layer is an electrode-position dependent value that varies according to the characteristic curve of the effective birefringence, and the curve has a convex shape as shown in FIG. 35(b). Therefore, the phase modulation amount of a light passing through this liquid crystal layer depends on the effective refractive index and, therefore, the light refracts. In this way, by applying a voltage to each signal electrode, the phase modulation amount of a light entering the liquid crystal optical modulator can be controlled and therefore this liquid crystal optical modulator can function as a convex lens.
The composite electrode 111 of this liquid crystal optical modulator, if configured as a circular electrode pattern, can be configured as a spherical lens. In FIG. 36, many semicircular stripe electrodes 3001–3008 are electrically connected by one gradient potential electrode 3010 with the center electrode 3009 as the center. The both ends of the gradient potential electrode 3010 are connected, respectively, to a first signal electrode 3031 and a second signal electrode 3033 that also function as a circular aperture. The stripe electrodes and the signal electrodes are separated into two areas by a first slit 3021 and a second slit 3023.
The operation of a spherical convex lens is the same as described above. In addition, by placing the gradient potential electrode along the diagonal line of the circular electrode pattern, a spherical lens with a rectangle aperture can be configured. In addition, a plurality of circular electrode patterns are arranged as an array to configure a micro lens array of variable-focal length type.
However, for the conventional liquid crystal optical modulator that employs a homogeneous alignment, the effective voltage applied to the liquid crystal is the lowest drive voltage in the vicinity of the center of the gradient potential electrode and, therefore, only the convex lens operation can be implemented in principle, especially for a spherical lens. In addition, when there is a need for the modulator to be included into a combination lens system, it is desirable that the focal point variable range be as large as possible. The problem with the conventional configuration is that the modulator finds uses only in an application where a narrow focal point range is acceptable because the operation is limited to the convex lens operation.